Imaging lens

ABSTRACT

An imaging lens includes a first lens group and a second lens group, arranged from an object side to an image plane side. The first lens group includes a first lens having positive refractive power, a second lens having positive refractive power, and a third lens having at least one aspheric surface, arranged with a space in between. The second lens group includes a fourth lens having positive refractive power and at least one aspheric surface, a fifth lens having two aspheric surfaces, and a sixth lens having two aspheric surfaces, arranged with a space in between. The fifth lens is formed in a shape so that a curvature radius of the surface on the image plane side is positive near an optical axis. The sixth lens is formed in a shape so that a curvature radius of the surface on the object side is negative near an optical axis.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of a prior application Ser. No. 15/131,111, filed on Apr. 18, 2016, pending, which is a continuation application of a prior application Ser. No. 14/568,173, filed on Dec. 12, 2014, issued on May 17, 2016 as U.S. Pat. No. 9,341,822, which claims priority of Japanese Patent Application No. 2014-136541, filed on Jul. 2, 2014.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for mounting in a relatively small camera such as a camera to be built in a portable device including a cellular phone and a portable information terminal, a digital still camera, a security camera, a vehicle onboard camera, and a network camera.

In these years, in place of cellular phones that are intended mainly for making phone calls, so-called “smartphones”, which are multi-functional cellular phones that can be used to execute various application software as well as making phone calls, have been more widely used. By executing application software on a smartphone, the smartphone can be used, for example, as a digital still camera, a car navigation system, or the like. In order to execute the functions, most models of the smartphones have cameras, similar to the cellular phones.

Generally speaking, product groups of the smartphones are often categorized according to specifications thereof for beginners to advanced users. Among them, an imaging lens to be mounted in a product designed for advanced users is required to have a high resolution lens configuration so as to be also applicable to a high pixel count imaging element.

As a method of attaining the high-resolution imaging lens, there is a method of increasing the number of lenses that compose the imaging lens. However, the increase of the number of lenses easily causes an increase in the size of the imaging lens. Therefore, the lens configuration having a large number of lenses is disadvantageous for mounting in a small-sized camera such as the above-described smartphones. For this reason, an imaging lens has been developed so as to restrain the number of lenses as small as possible. However, with rapid advancement in achieving higher pixel count of an imaging element in these days, an imaging lens has been developed so as to attain higher resolution rather than attaining a shorter total track length thereof. For example, although it has been common to mount a conventional camera unit containing an imaging lens and an imaging element inside of a smartphone, nowadays, there is an attempt to attach a camera unit, which is a separate unit from the smartphone, to a smartphone, so that it is possible to obtain an image that is equivalent to that obtained by a digital still camera.

In case of a lens configuration composed of six lenses, since the number of lenses that compose an imaging lens is large, it is somewhat disadvantageous for downsizing the imaging lens. However, since there is high flexibility in designing, it has potential of attaining satisfactory correction of aberrations and downsizing in a balanced manner. For example, as an imaging lens having a six-lens configuration as described above, an imaging lens described in Patent Reference has been known.

-   Patent Reference: Japanese Patent Application Publication No.     2013-195587

The imaging lens described in Patent Reference includes a first lens that is positive and has a convex surface directing to an object side, a second lens that is negative and has a concave surface directing to an image plane side, a third lens that is negative and has a concave surface directing to the object side, a fourth lens and a fifth lens that are positive and have convex surfaces to the image plane side, and a sixth lens that is negative and has a concave surface directing to the object side. According to the imaging lens of Patent Reference, by satisfying a conditional relation concerning a ratio between a focal length of the first lens and a focal length of the third lens and a ratio between a focal length of the second lens and a focal length of the whole lens system, it is possible to satisfactorily correct a distortion and a chromatic aberration.

The cellular phones and smartphones have higher functions and smaller sizes each year and imaging lenses are required to have even smaller sizes than before. In case of the imaging lens of Patent Reference, the distance from the object-side surface of the first lens to the image plane of an imaging element is long. Therefore, to satisfy the above-described demands, there is a limit by itself in satisfactorily correcting aberrations while downsizing the imaging lens.

Here, such a problem is not specific to the imaging lens to be mounted in the cellular phones and smartphones. Rather, it is a common problem even for an imaging lens to be mounted in a relatively small camera such as digital still cameras, portable information terminals, security cameras, vehicle onboard cameras, and network cameras.

In view of the above-described problems in conventional techniques, an object of the present invention is to provide an imaging lens that can attain downsizing thereof and satisfactory aberration correction.

Further objects and advantages of the present invention will be apparent from the following description of the invention.

SUMMARY OF THE INVENTION

In order to attain the objects described above, according to a first aspect of the present invention, an imaging lens includes a first lens group having positive refractive power; and a second lens group having negative refractive power, arranged in the order from an object side to an image plane side. The first lens group includes a first lens having positive refractive power, and a second lens having positive refractive power and a third lens having negative refractive power. The second lens group includes a fourth lens having positive refractive power, a fifth lens, and a sixth lens having negative refractive power.

According to the first aspect of the present invention, when the whole lens system has a focal length f, the first lens has a focal length f1, the first lens has an Abbe's number νd1, the second lens has an Abbe's number νd2, and the third lens has an Abbe's number νd3, the imaging lens of the present invention satisfies the following conditional expressions (1) to (4): 10<f1/f<40  (1) 40<νd1<75  (2) 40<νd2<75  (3) 15<νd3<35  (4)

According to the first aspect of the present invention, the first lens group is composed of three lenses, refractive powers of which are arranged in the order of positive-positive-negative. Those three lenses are made of lens materials that satisfy the conditional expressions (2) to (4). As a result, those three lenses are composed of a combination of lenses made of low-dispersion materials and a lens made of a high-dispersion material. With the arrangement of the refractive powers and the order of the Abbe's numbers of the respective lenses, it is possible to suitably restrain generation of a chromatic aberration in the first lens group and to satisfactorily correct the chromatic aberration if generated.

According to the first aspect of the invention, in the imaging lens, the positive refractive power is shared between the two lenses, the first lens and the second lens. Therefore, it is achievable to restrain the respective refractive powers of the first lens and the second lens to relatively small values, and it is achievable to suitably downsize the imaging lens while satisfactorily correcting aberrations.

When the imaging lens satisfies the conditional expression (1), it is achievable to satisfactorily correct an astigmatism and a field curvature, while downsizing the imaging lens. When the value exceeds the upper limit of “40”, the first lens has relatively weak positive refractive power relative to the refractive power of the whole lens system. As a result, the second lens has relatively strong positive refractive power in the first lens group. Therefore, an image-forming surface curves to the object side, i.e., the field curvature is insufficiently corrected. In addition, an astigmatic difference increases, so that it is difficult to obtain satisfactory image-forming performance.

On the other hand, when the value is below the lower limit of “10”, the first lens has relatively strong positive refractive power relative to the refractive power of the whole lens system. Therefore, it is advantageous for downsizing of the imaging lens. However, a back focal length is short, so that it is difficult to secure space for disposing an insert such as an infrared cutoff filter. Moreover, the astigmatism increases, and a coma aberration for off-axis light flux increases, so that it is difficult to obtain satisfactory image-forming performance.

According to a second aspect of the invention, when the first lens group has a focal length F1, the imaging lens having the above-described configuration preferably satisfies the following conditional expression (5): 0.7<F1/f<1.2  (5)

When the imaging lens satisfies the conditional expression (5), it is achievable to restrain the chromatic aberration and the astigmatism within satisfactory ranges, while downsizing the imaging lens. In addition, when the imaging lens satisfies the conditional expression (5), it is also achievable to restrain the incident angles of light beams emitted from the imaging lens to the image plane within the range of a chief ray angle (CRA). As is well known, an imaging element such as a CCD sensor or a CMOS sensor has a so-called chief ray angle (CRA) set in advance, i.e. a range of an incident angle of a light beam that can be taken in the sensor. By restraining the incident angle of a light beam emitted from the imaging lens to the image plane within the range of CRA, it is possible to suitably restrain generation of so-called “shading”, which is a phenomenon that the image periphery becomes dark.

When the value exceeds the upper limit of “1.2” in the conditional expression (5), the first lens group has weak refractive power relative to that of the whole lens system. Therefore, although it is advantageous for correction of an axial chromatic aberration, it is difficult to downsize the imaging lens. In addition, since the astigmatic difference increases at the image periphery, it is difficult to obtain satisfactory image-forming performance.

On the other hand, when the value is below the lower limit of “0.7”, the first lens group has strong refractive power relative to that of the whole lens system, so that it is advantageous for downsizing of the imaging lens. However, the axial chromatic aberration is insufficiently corrected (a focal position at a short wavelength moves to the object side relative to a focal position at a reference wavelength). At the same time, a chromatic aberration of magnification is excessively corrected (an image-forming point at a short wavelength moves in a direction to be away from an optical axis relative to an image-forming point at a reference wavelength). Moreover, since the field curvature is insufficiently corrected, so that it is difficult to obtain satisfactory image-forming performance. Furthermore, it is also difficult to restrain an incident angle of a light beam emitted from the imaging lens to the image plane within the range of CRA.

According to a third aspect of the invention, when the second lens has a focal length f2, the imaging lens having the above-described configuration preferably satisfies the following conditional expression (6): 0.3<f2/f<0.9  (6)

When the imaging lens satisfies the conditional expression (6), it is achievable to restrain the astigmatism and the chromatic aberration of magnification within satisfactory ranges, while downsizing the imaging lens. When the value exceeds the upper limit of “0.9”, the second lens has weak refractive power relative to that of the whole lens system, and the first lens group has relatively weak positive refractive power. For this reason, although it is advantageous for securing the back focal length, it is difficult to downsize the imaging lens.

As described above, the first lens group includes the first and the second lenses, which have positive refractive powers, and the third lens having negative refractive power. When the value exceeds the upper limit in the conditional expression (6), while the first lens group has relatively weak positive refractive power, the third lens has relatively strong negative refractive power. In order to satisfactorily correct aberrations in the first lens group, it is necessary to weaken the refractive power of the third lens. However, when the third lens has weak refractive power, the axial chromatic aberration is insufficiently corrected, and the astigmatic difference increases. Therefore, when the value exceeds the upper limit, it is difficult to obtain satisfactory image-forming performance.

On the other hand, when the value is below the lower limit of “0.3” in the conditional expression (6), the second lens has strong refractive power relative to the whole lens system, and the first lens group has relatively strong positive refractive power. Therefore, although it is advantageous for downsizing of the imaging lens, it is difficult to secure the back focal length. Here, when the value is below the lower limit, it is necessary to increase the refractive power of the third lens in order to satisfactorily correct the aberrations in the first lens group. In such a case, although it is advantageous for correcting the axial chromatic aberration, the chromatic aberration of magnification for an off-axis light flux is excessively corrected. Therefore, also when the value is below the lower limit, it is difficult to obtain satisfactory image-forming performance.

According to a fourth aspect of the invention, when the second lens has a focal length f2 and the third lens has a focal length f3, the imaging lens having the above-described configuration preferably satisfies the following conditional expression (7): −5<f3/f2<−1  (7)

When the imaging lens satisfies the conditional expression (7), it is achievable to satisfactorily correct the chromatic aberration and a spherical aberration, while downsizing the imaging lens. When the value exceeds the upper limit of “−1”, the third lens has strong negative refractive power relative to the positive refractive power of the second lens. Therefore, although it is advantageous for securing the back focal length, it is difficult to downsize the imaging lens. In addition, since a spherical aberration increases, it is difficult to obtain satisfactory image-forming performance. On the other hand, when the value is below the lower limit of “−5”, although it is advantageous for downsizing of the imaging lens, the axial chromatic aberration is insufficiently corrected. Therefore, also in this case, it is difficult to obtain satisfactory image-forming performance.

According to a fifth aspect of the invention, when the first lens group has a focal length F1 and the second lens group has a focal length F2, the imaging lens having the above-described configuration preferably satisfies the following conditional expression (8): −12<F2/F1<−1.5  (8)

When the imaging lens satisfies the conditional expression (8), it is achievable to restrain the astigmatism, the field curvature, and the chromatic aberration within preferable ranges in a balanced manner, while downsizing the imaging lens. When the value exceeds the upper limit of “−1.5”, it is advantageous for downsizing of the imaging lens. However, the axial chromatic aberration is insufficiently corrected and the chromatic aberration of magnification for an off-axis light flux at the image periphery is excessively corrected. Moreover, the astigmatic difference increases and the field curvature is insufficiently corrected, so that it is difficult to obtain satisfactory image-forming performance.

On the other hand, when the value is below the lower limit of “−12”, although it is easy to secure the back focal length, it is difficult to downsize the imaging lens. Moreover, the astigmatic difference increases, and the image-forming surface curves to the image plane side, i.e., the field curvature is excessively corrected. As a result, it is difficult to obtain satisfactory image-forming performance.

According to a sixth aspect of the invention, when a distance on an optical axis between the third lens and the fourth lens is D34, the imaging lens having the above-described configuration preferably satisfies the following conditional expression (9): 0.1<D34/f<0.4  (9)

When the imaging lens satisfies the conditional expression (9), it is achievable to restrain the distortion, the astigmatism, and the field curvature within preferred ranges in a balanced manner, while restraining an incident angle of a light beam emitted from the imaging lens to the image plane within the range of CRA. When the value exceeds the upper limit of “0.4”, although it is easy to restrain the incident angle within the range of CRA, it is difficult to secure the back focal length. In addition, the astigmatism increases, and the field curvature is excessively corrected, so that it is difficult to obtain satisfactory image-forming performance.

On the other hand, when the value is below the lower limit of “0.1”, a minus distortion increases and the field curvature is insufficiently corrected. Furthermore, since the astigmatism also increases, it is difficult to obtain satisfactory image-forming performance. Moreover, it is difficult to restrain an incident angle of a light beam emitted from the imaging lens to the image plane within the range of CRA.

According to a seventh aspect of the invention, when the fourth lens has an Abbe's number νd4, the fifth lens has an Abbe's number νd5, and the sixth lens has an Abbe's number νd6, in order to more satisfactorily correct the chromatic aberration, the imaging lens having the above-described configuration preferably satisfies the following conditional expressions (10) to (12): 40<νd4<75  (10) 15<νd5<35  (11) 40<νd6<75  (12)

According to an eighth aspect of the invention, when the fifth lens has negative refractive power, the fourth lens has a focal length f4, and the fifth lens has a focal length f5, the imaging lens having the above-described configuration preferably satisfies the following conditional expression (13): −15<f5/f4<−5  (13)

When the imaging lens satisfies the conditional expression (13), it is achievable to satisfactorily correct the chromatic aberration of magnification and the field curvature. When the value exceeds the upper limit of “−5”, the fifth lens has strong negative refractive power relative to the positive refractive power of the fourth lens. Therefore, the chromatic aberration of magnification for an off-axis light flux at an image periphery is excessively corrected, and the field curvature is insufficiently corrected. As a result, it is difficult to obtain satisfactory image-forming performance.

On the other hand, when the value is below the lower limit of “−15”, the fifth lens has weak negative refractive power relative to the positive refractive power of the fourth lens. In order to satisfactorily correct the aberrations, it is necessary to increase the refractive power of the sixth lens, which has also negative refractive power in the second lens group. However, in this case, although it is advantageous for correcting the field curvature, the chromatic aberration of magnification at the image periphery is excessively corrected. As a result, it is difficult to obtain satisfactory image-forming performance.

According to a ninth aspect of the invention, when a composite focal length of the fifth lens and the sixth lens is f56, the imaging lens having the above-described configuration preferably satisfies the following conditional expression (14): −3<f56/f<−0.8  (14)

When the imaging lens satisfies the conditional expression (14), it is achievable to restrain the chromatic aberration, the distortion, and the astigmatism within preferred ranges in a balanced manner, while downsizing the imaging lens. When the value exceeds the upper limit of “−0.8”, the second lens group has relatively strong negative refractive power. Although it is advantageous for downsizing of the imaging lens, a plus distortion increases and the chromatic aberration of magnification is excessively corrected at the image periphery. As a result, it is difficult to obtain satisfactory image-forming performance.

On the other hand, when the value is below the lower limit of “−3”, although it is easy to secure the back focal length, it is difficult to downsize the imaging lens. Moreover, the minus distortion increases and the astigmatic difference increases, so that it is difficult to obtain satisfactory image-forming performance.

According to a tenth aspect of the invention, when the sixth lens has a focal length f6 and a composite focal length of the fifth lens and the sixth lens is f56, the imaging lens having the above-described configuration preferably satisfies the following conditional expression (15): 0.7<f6/f56<1.2  (15)

When the imaging lens satisfies the conditional expression (15), it is achievable to restrain the chromatic aberration, the field curvature, and the distortion within preferred ranges in a balanced manner. As is shown in the conditional expression (15), according to the tenth aspect of the invention, the sixth lens accounts for the most part of the negative refractive power in the second lens group. The fifth lens has very weak refractive power relative to that of the sixth lens. With this configuration, it is achievable to perform fine correction of aberrations in the fifth lens, and it is achievable to suitably restrain the incident angle to the image plane within the range of CRA, in addition to correction of the aberrations, in the sixth lens.

When the value exceeds the upper limit of “1.2” in the conditional expression (15), although it is advantageous for correcting the axial chromatic aberration, the minus distortion increases. Moreover, the field curvature is insufficiently corrected, and the chromatic aberration of magnification is excessively corrected. As a result, it is difficult to obtain satisfactory image-forming performance.

On the other hand, when the value is below the lower limit of “0.7”, although it is easy to correct the distortion, the axial chromatic aberration is insufficiently corrected, and it is difficult to obtain satisfactory image-forming performance.

According to an eleventh aspect of the invention, when the sixth lens has a focal length f6, the imaging lens having the above-described configuration preferably satisfies the following conditional expression (16): −3.5<f6/f<−0.5  (16)

When the imaging lens satisfies the conditional expression (16), it is achievable to restrain the chromatic aberration, the distortion, and the astigmatism within preferred ranges in a balanced manner, while downsizing the imaging lens. Moreover, when the imaging lens satisfies the conditional expression (16), it is achievable to restrain an incident angle of a light beam emitted from the imaging lens to the image plane within the range of CRA. When the value exceeds the upper limit of “−0.5”, it is advantageous for correcting the axial chromatic aberration. However, the plus distortion increases, and the chromatic aberration of magnification for the off-axis light flux at the image periphery is excessively corrected, so that it is difficult to obtain satisfactory image-forming performance. In addition, it is difficult to restrain an incident angle of a light beam emitted from the imaging lens to the image plane within the range of CRA.

On the other hand, when the value is below the lower limit of “−3.5”, although it is easy to restrain the incident angle to the image plane within the range of CRA, the minus distortion increases and the chromatic aberration of magnification for the off-axis light flux at the image periphery is insufficiently corrected. As a result, it is also difficult to obtain satisfactory image-forming performance.

According to the imaging lens of the present invention, it is possible to provide a small-sized imaging lens that is especially suitable for mounting in a small-sized camera, while having high resolution with satisfactory correction of aberrations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of a schematic configuration of an imaging lens in Numerical Data Example 1 according to an embodiment of the present invention;

FIG. 2 is an aberration diagram showing a lateral aberration of the imaging lens of FIG. 1;

FIG. 3 is an aberration diagram showing a spherical aberration, astigmatism, and a distortion of the imaging lens of FIG. 1;

FIG. 4 shows a sectional view of a schematic configuration of an imaging lens in Numerical Data Example 2 according to the embodiment of the present invention;

FIG. 5 is an aberration diagram showing a lateral aberration of the imaging lens of FIG. 4;

FIG. 6 is an aberration diagram showing a spherical aberration, astigmatism, and a distortion of the imaging lens of FIG. 4;

FIG. 7 shows a sectional view of a schematic configuration of an imaging lens in Numerical Data Example 3 according to the embodiment of the present invention;

FIG. 8 is an aberration diagram showing a lateral aberration of the imaging lens of FIG. 7;

FIG. 9 is an aberration diagram showing a spherical aberration, astigmatism, and a distortion of the imaging lens of FIG. 7;

FIG. 10 shows a sectional view of a schematic configuration of an imaging lens in Numerical Data Example 4 according to the embodiment of the present invention;

FIG. 11 is an aberration diagram showing a lateral aberration of the imaging lens of FIG. 10;

FIG. 12 is an aberration diagram showing a spherical aberration, astigmatism, and a distortion of the imaging lens of FIG. 10;

FIG. 13 shows a sectional view of a schematic configuration of an imaging lens in Numerical Data Example 5 according to the embodiment of the present invention;

FIG. 14 is an aberration diagram showing a lateral aberration of the imaging lens of FIG. 13;

FIG. 15 is an aberration diagram showing a spherical aberration, astigmatism, and a distortion of the imaging lens of FIG. 13;

FIG. 16 shows a sectional view of a schematic configuration of an imaging lens in Numerical Data Example 6 according to the embodiment of the present invention;

FIG. 17 is an aberration diagram showing a lateral aberration of the imaging lens of FIG. 16;

FIG. 18 is an aberration diagram showing a spherical aberration, astigmatism, and a distortion of the imaging lens of FIG. 16;

FIG. 19 shows a sectional view of a schematic configuration of an imaging lens in Numerical Data Example 7 according to the embodiment of the present invention;

FIG. 20 is an aberration diagram showing a lateral aberration of the imaging lens of FIG. 19; and

FIG. 21 is an aberration diagram showing a spherical aberration, astigmatism, and a distortion of the imaging lens of FIG. 19.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereunder, referring to the accompanying drawings, an embodiment of the present invention will be fully described.

FIGS. 1, 4, 7, 10, 13, 16, and 19 are schematic sectional views of the imaging lenses in Numerical Data Examples 1 to 7 according to the embodiment, respectively. Since the imaging lenses in those Numerical Data Examples have the same basic configuration, the lens configuration of the embodiment will be described with reference to the illustrative sectional view of Numerical Data Example 1.

As shown in FIG. 1, according to the embodiment, the imaging lens includes a first lens group G1 having positive refractive power, and a second lens group G2 having negative refractive power, arranged in the order from an object side to an image plane side. Between the second lens group G2 and an image plane IM of an imaging element, there is provided a filter 10. The filter 10 is omissible.

The first lens group G1 includes a first lens L1 having positive refractive power, an aperture stop ST, a second lens L2 having positive refractive power, and a third lens L3 having negative refractive power, arranged in the order from the object side. According to the imaging lens of the embodiment, the aperture stop ST is provided on an image plane-side surface of the first lens L1. Here, where to provide the aperture stop ST is not limited to between the first lens L1 and the second lens L2 as in the imaging lens of Numerical Data Example 1.

For example, the aperture stop ST can be provided on the object side of the first lens L1. In case of a so-called “front aperture”-type lens configuration, in which the aperture stop ST is disposed on the object side of the first lens L1, it is achievable to improve assembling performance of the imaging lens and reduce the manufacturing cost. In case of the front aperture-type lens configuration, it is also relatively easy to shorten a total track length of the imaging lens, so that such lens configuration is effective for mounting in a portable device such as cellular phones and smartphones that are popular in these years.

On the other hand, in case of a so-called “mid aperture”-type lens configuration, in which the aperture stop ST is disposed between the first lens L1 and the second lens L2 as in Numerical Data Example 1, an effective diameter of the first lens L1 relative to the total track length of the imaging lens is large. Therefore, visual impact of the imaging lens in a camera is emphasized, so that it is possible to appeal luxuriousness, high lens performance, etc. to users as a part of design of the camera.

In the first lens group G1, the first lens L1 is formed in a shape, such that a curvature radius r1 of an object-side surface thereof and a curvature radius r2 of an image plane-side surface thereof are both positive, so as to have a shape of a meniscus lens directing a convex surface thereof to the object side near the optical axis X. The shape of the first lens L1 is not limited to the one in Numerical Data Example 1. The first lens L1 can be formed in any shape as long as the curvature radius r1 of the object-side surface is positive. More specifically, the first lens L1 can be also formed in a shape such that the curvature radius r2 is negative, so as to have a shape of a biconvex lens near an optical axis X. Here, in order to more effectively attain downsizing of the imaging lens, the first lens L1 is preferably formed so as to have a shape of a meniscus lens directing a convex surface thereof to the object side near the optical axis X.

The second lens L2 is formed in a shape such that a curvature radius r3 of an object-side surface thereof is positive and a curvature radius r4 of an image plane-side surface thereof is negative, so as to have a shape of a biconvex lens near the optical axis X. The shape of the second lens L2 is not limited to the one in Numerical Data Example 1. The second lens L2 can be formed in any shape as long as the curvature radius r4 of the image plane-side surface is negative. More specifically, the second lens L2 can be also formed in a shape such that the curvature radius r3 of the object-side surface thereof is negative, so as to have a shape of a meniscus lens directing a concave surface thereof to the object side near the optical axis X. Here, generally speaking, when the first lens L1 is formed so as to have a shape of a biconvex lens near the optical axis X, the second lens L2 is preferably formed so as to have a shape of a meniscus lens directing a concave surface thereof to the object side near the optical axis X.

The third lens L3 is formed in a shape such that a curvature radius r5 of an object-side surface thereof and a curvature radius r6 of an image plane-side surface thereof are both positive, so as to have a shape of a meniscus lens directing a convex surface thereof to the object side near the optical axis X. The shape of the third lens L3 is not limited to the one in Numerical Data Example 1 and can be any as long as the curvature radius r6 of the image plane-side surface thereof is positive. More specifically, the third lens L3 can be also formed in a shape, such that the curvature radius r5 of the object-side surface thereof is negative, i.e., a shape of a biconcave lens near the optical axis X.

The second lens group G2 includes a fourth lens L4 having positive refractive power, a fifth lens L5 having negative or positive refractive power, and a sixth lens L6 having negative refractive power, arranged in the order from the object side.

In the second lens group G2, the fourth lens L4 is formed in a shape such that a curvature radius r7 of an object-side surface thereof and a curvature radius r8 of an image plane-side surface thereof are both negative, so as to have a meniscus lens directing a concave surface thereof to the object side near the optical axis X. The fifth lens L5 is formed in a shape such that a curvature radius r9 of an object-side surface thereof and a curvature radius r10 of an image plane-side surface thereof are both positive, so as to have a shape of a meniscus lens directing a convex surface thereof to the object side near the optical axis X. The fifth lens L5 has the weakest refractive power in the second lens group G2. The imaging lenses in Numerical Data Examples 1 to 6 are examples of a lens configuration, in which the fifth lens L5 has negative refractive power. The imaging lens in Numerical Data Example 7 is an example, in which the fifth lens L5 has positive refractive power.

The sixth lens L6 is formed in a shape, such that a curvature radius r11 of an object-side surface thereof and a curvature radius r12 of an image plane-side surface thereof are both negative, so as to have a shape of a meniscus lens directing a concave surface thereof to the object side near the optical axis X. The shape of the sixth lens L6 is not limited to the one in Numerical Data Example 1, and can be any as long as the curvature radius r11 of the object-side surface thereof is negative. More specifically, the sixth lens L6 can be also formed in a shape such that the curvature radius r12 is positive, so as to have a shape of a biconcave lens near the optical axis X.

The fifth lens L5 and the sixth lens L6 are both formed such that the object-side surfaces thereof and the image plane-side surfaces thereof are formed as both aspheric surfaces, and the positive refractive powers thereof become stronger towards the lens peripheries. Due to the shapes of the fifth lens L5 and the sixth lens L6, it is achievable to satisfactorily correct off-axis chromatic aberrations of magnification as well as axial chromatic aberrations, and also achievable to suitably restrain the incident angles of light beams emitted from the imaging lens to the image plane IM within the range of chief ray angle (CRA).

According to the embodiment, the imaging lens satisfies the following conditional expressions (1) to (16): 10<f1/f<40  (1) 40<νd1<75  (2) 40<νd2<75  (3) 15<νd3<35  (4) 0.7<F1/f<1.2  (5) 0.3<f2/f<0.9  (6) −5<f3/f2<−1  (7) −12<F2/F1<−1.5  (8) 0.1<D34/f<0.4  (9) 40<νd4<75  (10) 15<νd5<35  (11) 40<νd6<75  (12) −15<f5/f4<−5  (13) −3<f56/f<−0.8  (14) 0.7<f6/f56<1.2  (15) −3.5<f6/f<−0.5  (16)

In the above conditional expressions:

-   f: Focal length of the whole lens system -   F1: Focal length of the first lens group G1 -   F2: Focal length of the second lens group G2 -   f1: Focal length of the first lens L1 -   f2: Focal length of the second lens L2 -   f3: Focal length of the third lens L3 -   f4: Focal length of the fourth lens L4 -   f5: Focal length of the fifth lens L5 -   f6: Focal length of the sixth lens L6 -   f56: Composite focal length of the fifth lens L5 and the sixth lens     L6 -   D34: Distance on the optical axis X between the third lens L3 and     the fourth lens L4 -   νd1: Abbe's number of the first lens L1 -   νd2: Abbe's number of the second lens L2 -   νd3: Abbe's number of the third lens L3 -   νd4: Abbe's number of the fourth lens L4 -   νd5: Abbe's number of the fifth lens L5 -   νd6: Abbe's number of the sixth lens L6

Here, it is not necessary to satisfy all of the conditional expressions, and it is achievable to obtain an effect corresponding to the respective conditional expression when any single one of the conditional expressions is individually satisfied.

In the embodiment, all lens surfaces are formed as an aspheric surface. When the aspheric shapes applied to the lens surfaces have an axis Z in a direction of the optical axis X, a height H in a direction perpendicular to the optical axis X, a conic constant k, and aspheric coefficients A₄, A₆, A₈, A₁₀, A₁₂, A₁₄, and A₁₆, the aspheric shapes of the lens surfaces are expressed as follows:

$\begin{matrix} {Z = {\frac{\frac{H^{2}}{R}}{1 + \sqrt{1 - {\left( {k + 1} \right)\frac{H^{2}}{R^{2}}}}} + {A_{4}H^{4}} + {A_{6}H^{6}} + {A_{8}H^{8}} + {A_{10}H^{10}} + {A_{12}H^{12}} + {A_{14}H^{14}} + {A_{16}H^{16}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack \end{matrix}$

Next, Numerical Data Examples of the imaging lens of the embodiment will be described. In each Numerical Data Example, f represents a focal length of the whole lens system, Fno represents an F-number, and ω represents a half angle of view, respectively. In addition, i represents a surface number counted from the object side, r represents a curvature radius, d represents a distance on the optical axis between lens surfaces (surface spacing), nd represents a refractive index, and νd represents an Abbe's number, respectively. Here, aspheric surfaces are indicated with surface numbers i affixed with * (asterisk).

Numerical Data Example 1

Basic data are shown below. f = 4.72 mm, Fno = 2.3, ω = 36.6° Unit: mm Surface Data Surface Number i r d nd νd (Object) ∞ ∞ 1* 2.540 0.322 1.5346 56.1 (=νd1) 2* (Stop) 2.678 0.112 3* 2.631 0.705 1.5346 56.1 (=νd2) 4* −3.753 0.030 5* 13.124 0.250 1.6355 24.0 (=νd3) 6* 2.910 0.954 (=D34) 7* −2.274 0.465 1.5346 56.1 (=νd4) 8* −1.665 0.128 9* 2.594 0.600 1.6142 26.0 (=νd5) 10*  2.271 0.415 11*  −2.621 0.490 1.5346 56.1 (=νd6) 12*  −43.371 0.040 13  ∞ 0.300 1.5168 64.2 14  ∞ 0.818 (Image ∞ plane) Aspheric Surface Data First Surface k = 0.000, A₄ = −8.594E−02, A₆ = −4.958E−03, A₈ = −6.808E−02, A₁₀ = 4.724E−02, A₁₂ = 7.098E−05, A₁₄ = −1.463E−02, A₁₆ = 4.207E−03 Second Surface k = 0.000, A₄ = −8.107E−02, A₆ = −9.566E−02, A₈ = −2.004E−02, A₁₀ = 2.865E−02, A₁₂ = 5.229E−02, A₁₄ = −5.644E−02, A₁₆ = 1.381E−02 Third Surface k = 0.000, A₄ = 3.973E−02, A₆ = −5.155E−02, A₈ = 1.130E−03, A₁₀ = 6.108E−03, A₁₂ = 1.955E−02, A₁₄ = −1.407E−02, A₁₆ = −9.526E−04 Fourth Surface k = 0.000, A₄ = 3.136E−02, A₆ = −2.899E−03, A₈ = 2.852E−03, A₁₀ = −1.870E−02, A₁₂ = 4.596E−03, A₁₄ = −7.942E−05, A₁₆ = −5.084E−04 Fifth Surface k = 0.000, A₄ = −1.519E−01, A₆ = 2.580E−02, A₈ = −4.738E−04, A₁₀ = 1.868E−03, A₁₂ = 1.413E−02, A₁₄ = −9.547E−03, A₁₆ = 2.189E−03 Sixth Surface k = 0.000, A₄ = −1.511E−01, A₆ = 4.516E−02, A₈ = −1.755E−03, A₁₀ = 2.979E−04, A₁₂ = −4.743E−03, A₁₄ = 7.871E−03, A₁₆ = −2.368E−03 Seventh Surface k = 0.000, A₄ = 1.374E−01, A₆ = −9.270E−02, A₈ = 7.188E−02, A₁₀ = −7.831E−02, A₁₂ = 5.426E−02, A₁₄ = −2.370E−02, A₁₆ = 3.688E−03 Eighth Surface k = 0.000, A₄ = 7.577E−02, A₆ = 2.107E−02, A₈ = −4.554E−03, A₁₀ = 2.116E−03, A₁₂ = −5.293E−03, A₁₄ = 1.770E−03, A₁₆ = 2.075E−05 Ninth Surface k = 0.000, A₄ = −1.573E−01, A₆ = 3.006E−02, A₈ = 2.872E−03, A₁₀ = −7.123E−03, A₁₂ = 1.359E−03, A₁₄ = 2.254E−04, A₁₆ = −5.339E−05 Tenth Surface k = 0.000, A₄ = −1.770E−01, A₆ = 4.465E−02, A₈ = −1.142E−02, A₁₀ = 1.116E−03, A₁₂ = 2.618E−04, A₁₄ = −6.690E−05, A₁₆ = 3.254E−06 Eleventh Surface k = 0.000, A₄ = −1.005E−02, A₆ = 1.677E−02, A₈ = −3.286E−03, A₁₀ = 5.634E−04, A₁₂ = −7.823E−05, A₁₄ = 2.687E−06, A₁₆ = 3.551E−07 Twelfth Surface k = 0.000, A₄ = −3.064E−02, A₆ = 2.210E−02, A₈ = −5.737E−03, A₁₀ = 5.547E−04, A₁₂ = 1.202E−05, A₁₄ = −5.825E−06, A₁₆ = 2.882E−07 f1 = 50.84 mm f2 = 3.01 mm f3 = −5.94 mm f4 = 9.18 mm f5 = −100.87 mm f6 = −5.24 mm f56 = −5.41 mm F1 = 4.90 mm F2 = −17.03 mm The values of the respective conditional expressions are as follows:

-   f1/f=10.78 -   F1/f=1.04 -   f2/f=0.64 -   f3/f2=−1.97 -   F2/F1=−3.48 -   D34/f=0.20 -   f5/f4=−10.99 -   f56/f=−1.15 -   f6/f56=0.97 -   f6/f=−1.11

Accordingly, the imaging lens of Numerical Data Example 1 satisfies the above-described conditional expressions. The distance on the optical axis X from the object-side surface of the first lens L1 to the image plane IM (length without the filter 10) is 5.53 mm, and downsizing of the imaging lens is attained.

FIG. 2 shows a lateral aberration that corresponds to a ratio H of each image height to the maximum image height (hereinafter referred to as “image height ratio H”), which is divided into a tangential direction and a sagittal direction (The same is true for FIGS. 5, 8, 11, 14, 17, and 20), in the imaging lens of Numerical Data Example 1. Furthermore, FIG. 3 shows a spherical aberration (mm), astigmatism (mm), and a distortion (%), respectively. In the astigmatism diagram, an aberration on a sagittal image surface S and an aberration on a tangential image surface T are respectively indicated (The same is true for FIGS. 6, 9, 12, 15, 18, and 21). As shown in FIGS. 2 and 3, according to the imaging lens of Numerical Data Example 1, the aberrations are satisfactorily corrected.

Numerical Data Example 2

Basic data are shown below. f = 4.17 mm, Fno = 2.2, ω = 40.0° Unit: mm Surface Data Surface Number i r d nd νd (Object) ∞ ∞ 1* 2.640 0.301 1.5346 56.1 (=νd1) 2* (Stop) 2.623 0.100 3* 2.548 0.731 1.5346 56.1 (=νd2) 4* −3.600 0.030 5* 12.413 0.250 1.6355 24.0 (=νd3) 6* 3.003 0.827 (=D34) 7* −2.554 0.502 1.5346 56.1 (=νd4) 8* −1.644 0.008 9* 2.582 0.628 1.6142 26.0 (=νd5) 10*  2.251 0.430 11*  −2.619 0.490 1.5346 56.1 (=νd6) 12*  −45.373 0.040 13  ∞ 0.300 1.5168 64.2 14  ∞ 0.695 (Image ∞ plane) Aspheric Surface Data First Surface k = 0.000, A₄ = −9.029E−02, A₆ = −3.009E−03, A₈ = −6.611E−02, A₁₀ = 4.776E−02, A₁₂ = −2.396E−05, A₁₄ = −1.472E−02, A₁₆ = 4.271E−03 Second Surface k = 0.000, A₄ = −8.079E−02, A₆ = −9.490E−02, A₈ = −1.944E−02, A₁₀ = 2.897E−02, A₁₂ = 5.234E−02, A₁₄ = −5.654E−02, A₁₆ = 1.371E−02 Third Surface k = 0.000, A₄ = 3.895E−02, A₆ = −5.510E−02, A₈ = 6.493E−05, A₁₀ = 5.932E−03, A₁₂ = 1.950E−02, A₁₄ = −1.410E−02, A₁₆ = −9.727E−04 Fourth Surface k = 0.000, A₄ = 2.831E−02, A₆ = −2.611E−03, A₈ = 3.332E−03, A₁₀ = −1.855E−02, A₁₂ = 4.623E−03, A₁₄ = −6.681E−05, A₁₆ = −4.902E−04 Fifth Surface k = 0.000, A₄ = −1.498E−01, A₆ = 2.642E−02, A₈ = −6.170E−04, A₁₀ = 1.760E−03, A₁₂ = 1.409E−02, A₁₄ = −9.566E−03, A₁₆ = 2.152E−03 Sixth Surface k = 0.000, A₄ = −1.477E−01, A₆ = 4.201E−02, A₈ = −2.268E−03, A₁₀ = 1.714E−04, A₁₂ = −4.848E−03, A₁₄ = 7.816E−03, A₁₆ = −2.383E−03 Seventh Surface k = 0.000, A₄ = 1.460E−01, A₆ = −9.229E−02, A₈ = 7.149E−02, A₁₀ = −7.807E−02, A₁₂ = 5.476E−02, A₁₄ = −2.324E−02, A₁₆ = 3.971E−03 Eighth Surface k = 0.000, A₄ = 7.413E−02, A₆ = 2.130E−02, A₈ = −4.373E−03, A₁₀ = 2.217E−03, A₁₂ = −5.217E−03, A₁₄ = 1.817E−03, A₁₆ = 4.705E−05 Ninth Surface k = 0.000, A₄ = −1.645E−01, A₆ = 2.898E−02, A₈ = 1.460E−03, A₁₀ = −7.751E−03, A₁₂ = 1.221E−03, A₁₄ = 2.346E−04, A₁₆ = −3.068E−05 Tenth Surface k = 0.000, A₄ = −1.742E−01, A₆ = 4.358E−02, A₈ = −1.132E−02, A₁₀ = 1.136E−03, A₁₂ = 2.605E−04, A₁₄ = −6.787E−05, A₁₆ = 3.043E−06 Eleventh Surface k = 0.000, A₄ = −1.212E−02, A₆ = 1.702E−02, A₈ = −3.251E−03, A₁₀ = 5.657E−04, A₁₂ = −7.834E−05, A₁₄ = 2.636E−06, A₁₆ = 3.396E−07 Twelfth Surface k = 0.000, A₄ = −3.082E−02, A₆ = 2.153E−02, A₈ = −5.713E−03, A₁₀ = 5.587E−04, A₁₂ = 1.227E−05, A₁₄ = −5.832E−06, A₁₆ = 2.817E−07 f1 = 145.98 mm f2 = 2.91 mm f3 = −6.30 mm f4 = 7.23 mm f5 = −102.11 mm f6 = −5.22 mm f56 = −5.42 mm F1 = 4.72 mm F2 = −46.16 mm The values of the respective conditional expressions are as follows:

-   f1/f=34.98 -   F1/f=1.13 -   f2/f=0.70 -   f3/f2=−2.16 -   F2/F1=−9.78 -   D34/f=0.20 -   f5/f4=−14.11 -   f56/f=−1.30 -   f6/f56=0.96 -   f6/f=−1.25

Accordingly, the imaging lens of Numerical Data Example 2 satisfies the above-described conditional expressions. The distance on the optical axis X from the object-side surface of the first lens L1 to the image plane IM (length without the filter 10) is 5.23 mm, and downsizing of the imaging lens is attained.

FIG. 5 shows a lateral aberration that corresponds to the image height ratio H of the imaging lens, and FIG. 6 shows a spherical aberration (mm), astigmatism (mm), and a distortion (%), respectively, of the imaging lens of Numerical Data Example 2. As shown in FIGS. 5 and 6, according to the imaging lens of Numerical Data Example 2, the aberrations are also satisfactorily corrected.

Numerical Data Example 3

Basic data are shown below. f = 5.57 mm, Fno = 3.0, ω = 32.1° Unit: mm Surface Data Surface Number i r d nd νd (Object) ∞ ∞ 1* 2.411 0.310 1.5346 56.1 (=νd1) 2* (Stop) 2.411 0.120 3* 2.584 0.786 1.5346 56.1 (=νd2) 4* −3.786 0.030 5* 13.454 0.250 1.6355 24.0 (=νd3) 6* 2.712 0.651 (=D34) 7* −2.286 0.619 1.5346 56.1 (=νd4) 8* −1.734 0.066 9* 2.736 0.534 1.6142 26.0 (=νd5) 10*  2.425 0.377 11*  −2.777 0.490 1.5346 56.1 (=νd6) 12*  −10.661 0.040 13  ∞ 0.300 1.5168 64.2 14  ∞ 1.829 (Image ∞ plane) Aspheric Surface Data First Surface k = 0.000, A₄ = −8.574E−02, A₆ = −2.408E−03, A₈ = −6.584E−02, A₁₀ = 4.778E−02, A₁₂ = −1.745E−04, A₁₄ = −1.483E−02, A₁₆ = 4.339E−03 Second Surface k = 0.000, A₄ = −8.011E−02, A₆ = −9.318E−02, A₈ = −1.939E−02, A₁₀ = 2.819E−02, A₁₂ = 5.150E−02, A₁₄ = −5.701E−02, A₁₆ = 1.391E−02 Third Surface k = 0.000, A₄ = 3.737E−02, A₆ = −5.662E−02, A₈ = 9.497E−04, A₁₀ = 6.629E−03, A₁₂ = 1.907E−02, A₁₄ = −1.547E−02, A₁₆ = −2.843E−03 Fourth Surface k = 0.000, A₄ = 3.028E−02, A₆ = −4.810E−03, A₈ = 1.480E−03, A₁₀ = −1.931E−02, A₁₂ = 4.487E−03, A₁₄ = −1.826E−05, A₁₆ = −4.979E−04 Fifth Surface k = 0.000, A₄ = −1.489E−01, A₆ = 2.903E−02, A₈ = 8.808E−04, A₁₀ = 2.428E−03, A₁₂ = 1.435E−02, A₁₄ = −9.417E−03, A₁₆ = 2.330E−03 Sixth Surface k = 0.000, A₄ = −1.486E−01, A₆ = 3.921E−02, A₈ = −3.126E−03, A₁₀ = 3.864E−05, A₁₂ = −4.673E−03, A₁₄ = 8.094E−03, A₁₆ = −2.132E−03 Seventh Surface k = 0.000, A₄ = 1.493E−01, A₆ = −9.462E−02, A₈ = 6.796E−02, A₁₀ = −7.982E−02, A₁₂ = 5.416E−02, A₁₄ = −2.336E−02, A₁₆ = 3.969E−03 Eighth Surface k = 0.000, A₄ = 7.146E−02, A₆ = 2.321E−02, A₈ = −3.123E−03, A₁₀ = 2.538E−03, A₁₂ = −5.186E−03, A₁₄ = 1.796E−03, A₁₆ = 2.765E−05 Ninth Surface k = 0.000, A₄ = −1.612E−01, A₆ = 2.896E−02, A₈ = 2.664E−03, A₁₀ = −7.024E−03, A₁₂ = 1.339E−03, A₁₄ = 1.967E−04, A₁₆ = −6.638E−05 Tenth Surface k = 0.000, A₄ = −1.729E−01, A₆ = 4.397E−02, A₈ = −1.147E−02, A₁₀ = 1.109E−03, A₁₂ = 2.599E−04, A₁₄ = −6.764E−05, A₁₆ = 2.959E−06 Eleventh Surface k = 0.000, A₄ = −1.458E−02, A₆ = 1.675E−02, A₈ = −3.260E−03, A₁₀ = 5.684E−04, A₁₂ = −7.722E−05, A₁₄ = 2.945E−06, A₁₆ = 4.082E−07 Twelfth Surface k = 0.000, A₄ = −2.949E−02, A₆ = 2.146E−02, A₈ = −5.720E−03, A₁₀ = 5.604E−04, A₁₂ = 1.266E−05, A₁₄ = −5.799E−06, A₁₆ = 2.795E−07 f1 = 100.56 mm f2 = 3.00 mm f3 = −5.39 mm f4 = 9.66 mm f5 = −100.49 mm f6 = −7.18 mm f56 = −7.18 mm F1 = 5.45 mm F2 = −32.86 mm The values of the respective conditional expressions are as follows:

-   f1/f=18.04 -   F1/f=0.98 -   f2/f=0.54 -   f3/f2=−1.80 -   F2/F1=−6.03 -   D34/f=0.18 -   f5/f4=−10.40 -   f56/f=−1.29 -   f6/f56=1.00 -   f6/f=−1.29

Accordingly, the imaging lens of Numerical Data Example 3 satisfies the above-described conditional expressions. The distance on the optical axis X from the object-side surface of the first lens L1 to the image plane IM (length without the filter 10) is 6.30 mm, and downsizing of the imaging lens is attained.

FIG. 8 shows a lateral aberration that corresponds to the image height ratio H of the imaging lens, and FIG. 9 shows a spherical aberration (mm), astigmatism (mm), and a distortion (%), respectively, of the imaging lens of Numerical Data Example 3. As shown in FIGS. 8 and 9, according to the imaging lens of Numerical Data Example 3, the aberrations are also satisfactorily corrected.

Numerical Data Example 4

Basic data are shown below. f = 5.49 mm, Fno = 2.7, ω = 32.5° Unit: mm Surface Data Surface Number i r d nd νd (Object) ∞ ∞ 1* 2.919 0.311 1.5346 56.1 (=νd1) 2* (Stop) 2.972 0.059 3* 3.227 0.597 1.5346 56.1 (=νd2) 4* −3.911 0.030 5* 11.449 0.250 1.6355 24.0 (=νd3) 6* 3.477 1.824 (=D34) 7* −2.256 0.328 1.5346 56.1 (=νd4) 8* −1.706 0.010 9* 2.568 0.520 1.6142 26.0 (=νd5) 10*  2.272 0.420 11*  −2.623 0.490 1.5346 56.1 (=νd6) 12*  −108.154 0.040 13  ∞ 0.300 1.5168 64.2 14  ∞ 0.915 (Image ∞ plane) Aspheric Surface Data First Surface k = 0.000, A₄ = −8.446E−02, A₆ = 2.265E−03, A₈ = −6.239E−02, A₁₀ = 4.824E−02, A₁₂ = −3.062E−04, A₁₄ = −1.524E−02, A₁₆ = 4.143E−03 Second Surface k = 0.000, A₄ = −7.225E−02, A₆ = −8.484E−02, A₈ = −1.710E−02, A₁₀ = 2.743E−02, A₁₂ = 5.023E−02, A₁₄ = −5.745E−02, A₁₆ = 1.454E−02 Third Surface k = 0.000, A₄ = 4.777E−02, A₆ = −5.361E−02, A₈ = 1.061E−03, A₁₀ = 6.257E−03, A₁₂ = 1.947E−02, A₁₄ = −1.491E−02, A₁₆ = −2.018E−03 Fourth Surface k = 0.000, A₄ = 2.522E−02, A₆ = −1.962E−03, A₈ = 5.473E−03, A₁₀ = −1.738E−02, A₁₂ = 4.681E−03, A₁₄ = −7.421E−04, A₁₆ = −1.111E−03 Fifth Surface k = 0.000, A₄ = −1.372E−01, A₆ = 2.810E−02, A₈ = −1.443E−03, A₁₀ = 9.473E−04, A₁₂ = 1.368E−02, A₁₄ = −9.705E−03, A₁₆ = 1.879E−03 Sixth Surface k = 0.000, A₄ = −1.405E−01, A₆ = 5.035E−02, A₈ = −5.014E−04, A₁₀ = 1.135E−04, A₁₂ = −5.309E−03, A₁₄ = 7.228E−03, A₁₆ = −2.740E−03 Seventh Surface k = 0.000, A₄ = 1.023E−01, A₆ = −8.409E−02, A₈ = 7.439E−02, A₁₀ = −7.942E−02, A₁₂ = 5.363E−02, A₁₄ = −2.355E−02, A₁₆ = 4.053E−03 Eighth Surface k = 0.000, A₄ = 7.354E−02, A₆ = 1.479E−02, A₈ = −6.064E−03, A₁₀ = 1.766E−03, A₁₂ = −5.416E−03, A₁₄ = 1.719E−03, A₁₆ = 3.610E−06 Ninth Surface k = 0.000, A₄ = −1.471E−01, A₆ = 3.398E−02, A₈ = 2.887E−03, A₁₀ = −7.167E−03, A₁₂ = 1.329E−03, A₁₄ = 2.112E−04, A₁₆ = −5.826E−05 Tenth Surface k = 0.000, A₄ = −1.794E−01, A₆ = 4.560E−02, A₈ = −1.140E−02, A₁₀ = 1.081E−03, A₁₂ = 2.544E−04, A₁₄ = −6.744E−05, A₁₆ = 3.371E−06 Eleventh Surface k = 0.000, A₄ = −1.180E−02, A₆ = 1.648E−02, A₈ = −3.280E−03, A₁₀ = 5.656E−04, A₁₂ = −7.795E−05, A₁₄ = 2.670E−06, A₁₆ = 3.494E−07 Twelfth Surface k = 0.000, A₄ = −2.650E−02, A₆ = 2.137E−02, A₈ = −5.791E−03, A₁₀ = 5.614E−04, A₁₂ = 1.252E−05, A₁₄ = −5.831E−06, A₁₆ = 2.817E−07 f1 = 100.47 mm f2 = 3.41 mm f3 = −7.95 mm f4 = 10.84 mm f5 = −96.67 mm f6 = −5.04 mm f56 = −5.14 mm F1 = 5.31 mm F2 = −11.14 mm The values of the respective conditional expressions are as follows:

-   f1/f=18.30 -   F1/f=0.97 -   f2/f=0.62 -   f3/f2=−2.33 -   F2/F1=−2.10 -   D34/f=0.33 -   f5/f4=−8.91 -   f56/f=−0.94 -   f6/f56=0.98 -   f6/f=−0.92

Accordingly, the imaging lens of Numerical Data Example 4 satisfies the above-described conditional expressions. The distance on the optical axis X from the object-side surface of the first lens L1 to the image plane IM (length without the filter 10) is 5.99 mm, and downsizing of the imaging lens is attained.

FIG. 11 shows a lateral aberration that corresponds to the image height ratio H of the imaging lens, and FIG. 12 shows a spherical aberration (mm), astigmatism (mm), and a distortion (%), respectively, of the imaging lens of Numerical Data Example 4. As shown in FIGS. 11 and 12, according to the imaging lens of Numerical Data Example 4, the aberrations are also satisfactorily corrected.

Numerical Data Example 5

Basic data are shown below. f = 4.78 mm, Fno = 2.3, ω = 36.2° Unit: mm Surface Data Surface Number i r d nd νd (Object) ∞ ∞ 1* 2.820 0.391 1.5346 56.1 (=νd1) 2* (Stop) 2.832 0.101 3* 3.266 0.626 1.5346 56.1 (=νd2) 4* −4.128 0.030 5* 8.443 0.250 1.6355 24.0 (=νd3) 6* 4.195 1.266 (=D34) 7* −2.233 0.373 1.5346 56.1 (=νd4) 8* −1.728 0.010 9* 2.598 0.518 1.6142 26.0 (=νd5) 10*  2.263 0.425 11*  −2.612 0.490 1.5346 56.1 (=νd6) 12*  −325.335 0.040 13  ∞ 0.300 1.5168 64.2 14  ∞ 0.803 (Image ∞ plane) Aspheric Surface Data First Surface k = 0.000, A₄ = −8.756E−02, A₆ = 6.966E−03, A₈ = −6.001E−02, A₁₀ = 4.784E−02, A₁₂ = −9.244E−04, A₁₄ = −1.503E−02, A₁₆ = 5.154E−03 Second Surface k = 0.000, A₄ = −7.256E−02, A₆ = −8.029E−02, A₈ = −1.447E−02, A₁₀ = 2.822E−02, A₁₂ = 5.017E−02, A₁₄ = −5.749E−02, A₁₆ = 1.506E−02 Third Surface k = 0.000, A₄ = 5.347E−02, A₆ = −6.030E−02, A₈ = −1.115E−04, A₁₀ = 7.713E−03, A₁₂ = 2.072E−02, A₁₄ = −1.472E−02, A₁₆ = −2.467E−03 Fourth Surface k = 0.000, A₄ = 1.710E−02, A₆ = −1.536E−03, A₈ = 5.693E−03, A₁₀ = −1.730E−02, A₁₂ = 4.608E−03, A₁₄ = −8.975E−04, A₁₆ = −1.305E−03 Fifth Surface k = 0.000, A₄ = −1.388E−01, A₆ = 2.945E−02, A₈ = −1.784E−03, A₁₀ = 5.741E−04, A₁₂ = 1.364E−02, A₁₄ = −9.545E−03, A₁₆ = 2.093E−03 Sixth Surface k = 0.000, A₄ = −1.348E−01, A₆ = 4.218E−02, A₈ = −7.409E−04, A₁₀ = 1.474E−03, A₁₂ = −4.458E−03, A₁₄ = 7.595E−03, A₁₆ = −2.580E−03 Seventh Surface k = 0.000, A₄ = 1.215E−01, A₆ = −9.862E−02, A₈ = 7.588E−02, A₁₀ = −7.883E−02, A₁₂ = 5.363E−02, A₁₄ = −2.323E−02, A₁₆ = 4.465E−03 Eighth Surface k = 0.000, A₄ = 6.557E−02, A₆ = 1.334E−02, A₈ = −6.731E−03, A₁₀ = 2.039E−03, A₁₂ = −5.180E−03, A₁₄ = 1.818E−03, A₁₆ = 3.540E−05 Ninth Surface k = 0.000, A₄ = −1.767E−01, A₆ = 3.036E−02, A₈ = 3.829E−03, A₁₀ = −7.507E−03, A₁₂ = 1.041E−03, A₁₄ = 1.348E−04, A₁₆ = −4.949E−05 Tenth Surface k = 0.000, A₄ = −1.823E−01, A₆ = 4.478E−02, A₈ = −1.096E−02, A₁₀ = 1.154E−03, A₁₂ = 2.537E−04, A₁₄ = −6.993E−05, A₁₆ = 2.705E−06 Eleventh Surface k = 0.000, A₄ = −9.793E−03, A₆ = 1.721E−02, A₈ = −3.290E−03, A₁₀ = 5.562E−04, A₁₂ = −7.907E−05, A₁₄ = 2.557E−06, A₁₆ = 3.473E−07 Twelfth Surface k = 0.000, A₄ = −3.084E−02, A₆ = 2.134E−02, A₈ = −5.751E−03, A₁₀ = 5.647E−04, A₁₂ = 1.259E−05, A₁₄ = −5.851E−06, A₁₆ = 2.761E−07 f1 = 100.58 mm f2 = 3.51 mm f3 = −13.43 mm f4 = 11.38 mm f5 = −69.04 mm f6 = −4.93 mm f56 = −4.92 mm F1 = 4.52 mm F2 = −9.48 mm The values of the respective conditional expressions are as follows:

-   f1/f=21.02 -   F1/f=0.94 -   f2/f=0.73 -   f3/f2=−3.82 -   F2/F1=−2.10 -   D34/f=0.26 -   f5/f4=−6.07 -   f56/f=−1.03 -   f6/f56=1.00 -   f6/f=−1.03

Accordingly, the imaging lens of Numerical Data Example 5 satisfies the above-described conditional expressions. The distance on the optical axis X from the object-side surface of the first lens L1 to the image plane IM (length without the filter 10) is 5.52 mm, and downsizing of the imaging lens is attained.

FIG. 14 shows a lateral aberration that corresponds to the image height ratio H of the imaging lens, and FIG. 15 shows a spherical aberration (mm), astigmatism (mm), and a distortion (%), respectively, of the imaging lens of Numerical Data Example 5. As shown in FIGS. 14 and 15, according to the imaging lens of Numerical Data Example 5, the aberrations are also satisfactorily corrected.

Numerical Data Example 6

Basic data are shown below. f = 4.59 mm, Fno = 2.3, ω = 37.3° Unit: mm Surface Data Surface Number i r d nd νd (Object) ∞ ∞ 1* 2.394 0.324 1.5346 56.1 (=νd1) 2* (Stop) 2.506 0.078 3* 2.498 0.778 1.5346 56.1 (=νd2) 4* −4.033 0.030 5* 9.929 0.250 1.6355 24.0 (=νd3) 6* 2.749 0.751 (=D34) 7* −1.844 0.401 1.5346 56.1 (=νd4) 8* −1.669 0.232 9* 2.580 0.688 1.6142 26.0 (=νd5) 10*  2.218 0.581 11*  −2.659 0.490 1.5346 56.1 (=νd6) 12*  −4.576 0.040 13  ∞ 0.300 1.5168 64.2 14  ∞ 0.646 (Image ∞ plane) Aspheric Surface Data First Surface k = 0.000, A₄ = −7.447E−02, A₆ = 2.499E−03, A₈ = −7.205E−02, A₁₀ = 4.892E−02, A₁₂ = −5.711E−04, A₁₄ = −1.577E−02, A₁₆ = 5.190E−03 Second Surface k = 0.000, A₄ = −6.909E−02, A₆ = −9.439E−02, A₈ = −1.939E−02, A₁₀ = 2.926E−02, A₁₂ = 5.104E−02, A₁₄ = −5.806E−02, A₁₆ = 1.510E−02 Third Surface k = 0.000, A₄ = 2.859E−02, A₆ = −5.882E−02, A₈ = 3.748E−03, A₁₀ = 1.056E−02, A₁₂ = 1.872E−02, A₁₄ = −1.482E−02, A₁₆ = −1.925E−03 Fourth Surface k = 0.000, A₄ = 8.676E−03, A₆ = −2.236E−03, A₈ = 5.505E−03, A₁₀ = −1.880E−02, A₁₂ = 3.832E−03, A₁₄ = −7.443E−04, A₁₆ = −4.898E−04 Fifth Surface k = 0.000, A₄ = −1.455E−01, A₆ = 2.401E−02, A₈ = −6.875E−03, A₁₀ = 5.477E−04, A₁₂ = 1.407E−02, A₁₄ = −8.432E−03, A₁₆ = 1.879E−03 Sixth Surface k = 0.000, A₄ = −1.253E−01, A₆ = 2.536E−02, A₈ = 1.986E−03, A₁₀ = −3.142E−04, A₁₂ = −7.558E−03, A₁₄ = 7.643E−03, A₁₆ = −1.722E−03 Seventh Surface k = 0.000, A₄ = 1.443E−01, A₆ = −1.142E−01, A₈ = 8.684E−02, A₁₀ = −7.662E−02, A₁₂ = 4.838E−02, A₁₄ = −2.309E−02, A₁₆ = 4.006E−03 Eighth Surface k = 0.000, A₄ = 3.238E−02, A₆ = 2.005E−02, A₈ = −4.774E−03, A₁₀ = 3.102E−03, A₁₂ = −5.191E−03, A₁₄ = 2.182E−03, A₁₆ = 1.113E−04 Ninth Surface k = 0.000, A₄ = −1.806E−01, A₆ = 3.290E−02, A₈ = 2.012E−03, A₁₀ = −8.917E−03, A₁₂ = 1.206E−03, A₁₄ = 2.038E−04, A₁₆ = −2.055E−05 Tenth Surface k = 0.000, A₄ = −1.786E−01, A₆ = 4.410E−02, A₈ = −1.250E−02, A₁₀ = 1.273E−03, A₁₂ = 2.399E−04, A₁₄ = −7.472E−05, A₁₆ = 5.332E−06 Eleventh Surface k = 0.000, A₄ = −1.867E−02, A₆ = 1.627E−02, A₈ = −2.858E−03, A₁₀ = 4.966E−04, A₁₂ = −7.390E−05, A₁₄ = 2.900E−06, A₁₆ = 2.802E−07 Twelfth Surface k = 0.000, A₄ = −1.489E−02, A₆ = 2.173E−02, A₈ = −5.825E−03, A₁₀ = 5.278E−04, A₁₂ = 2.004E−05, A₁₄ = −6.555E−06, A₁₆ = 3.081E−07 f1 = 49.97 mm f2 = 3.01 mm f3 = −6.06 mm f4 = 18.28 mm f5 = −93.19 mm f6 = −13. 03 mm f56 = −12.54 mm F1 = 4 .76 mm F2 = −46.78 mm The values of the respective conditional expressions are as follows:

-   f1/f=10.88 -   F1/f=1.04 -   f2/f=0.66 -   f3/f2=−2.01 -   F2/F1=−9.82 -   D34/f=0.16 -   f5/f4=−5.10 -   f56/f=−2.73 -   f6/f56=1.04 -   f6/f=−2.84

Accordingly, the imaging lens of Numerical Data Example 6 satisfies the above-described conditional expressions. The distance on the optical axis X from the object-side surface of the first lens L1 to the image plane IM (length without the filter 10) is 5.49 mm, and downsizing of the imaging lens is attained.

FIG. 17 shows a lateral aberration that corresponds to the image height ratio H of the imaging lens, and FIG. 18 shows a spherical aberration (mm), astigmatism (mm), and a distortion (%), respectively, of the imaging lens of Numerical Data Example 6. As shown in FIGS. 17 and 18, according to the imaging lens of Numerical Data Example 6, the aberrations are also satisfactorily corrected.

Numerical Data Example 7

Basic data are shown below. f = 5.59 mm, Fno = 3.0, ω = 32.1° Unit: mm Surface Data Surface Number i r d nd νd (Object) ∞ ∞ 1* 2.543 0.343 1.5346 56.1 (=νd1) 2* (Stop) 2.557 0.108 3* 2.573 0.693 1.5346 56.1 (=νd2) 4* −3.368 0.046 5* 24.576 0.250 1.6355 24.0 (=νd3) 6* 3.170 0.674 (=D34) 7* −1.708 0.423 1.5346 56.1 (=νd4) 8* −1.798 0.348 9* 2.535 0.509 1.6142 26.0 (=νd5) 10*  2.440 0.544 11*  −2.749 0.405 1.5346 56.1 (=νd6) 12*  −6.157 0.040 13  ∞ 0.300 1.5168 64.2 14  ∞ 1.484 (Image ∞ plane) Aspheric Surface Data First Surface k = 0.000, A₄ = −8.425E−02, A₆ = −5.731E−03, A₈ = −6.587E−02, A₁₀ = 5.014E−02, A₁₂ = −1.499E−03, A₁₄ = −1.475E−02, A₁₆ = 4.102E−03 Second Surface k = 0.000, A₄ = −8.546E−02, A₆ = −9.353E−02, A₈ = −1.739E−02, A₁₀ = 2.901E−02, A₁₂ = 5.224E−02, A₁₄ = −5.801E−02, A₁₆ = 1.399E−02 Third Surface k = 0.000, A₄ = 3.398E−02, A₆ = −5.949E−02, A₈ = 3.317E−03, A₁₀ = 6.791E−03, A₁₂ = 1.762E−02, A₁₄ = −1.621E−02, A₁₆ = −2.591E−03 Fourth Surface k = 0.000, A₄ = 2.275E−02, A₆ = −4.423E−03, A₈ = 1.566E−03, A₁₀ = −2.000E−02, A₁₂ = 5.082E−03, A₁₄ = 5.211E−04, A₁₆ = −1.366E−04 Fifth Surface k = 0.000, A₄ = −1.541E−01, A₆ = 3.068E−02, A₈ = 2.597E−03, A₁₀ = 3.400E−03, A₁₂ = 1.510E−02, A₁₄ = −8.697E−03, A₁₆ = 3.304E−03 Sixth Surface k = 0.000, A₄ = −1.499E−01, A₆ = 3.668E−02, A₈ = −2.460E−03, A₁₀ = 3.961E−04, A₁₂ = −4.229E−03, A₁₄ = 7.933E−03, A₁₆ = −1.968E−03 Seventh Surface k = 0.000, A₄ = 1.518E−01, A₆ = −9.269E−02, A₈ = 6.624E−02, A₁₀ = −7.946E−02, A₁₂ = 5.477E−02, A₁₄ = −2.393E−02, A₁₆ = 3.482E−03 Eighth Surface k = 0.000, A₄ = 6.958E−02, A₆ = 2.231E−02, A₈ = −2.553E−03, A₁₀ = 3.319E−03, A₁₂ = −5.110E−03, A₁₄ = 1.803E−03, A₁₆ = 2.276E−05 Ninth Surface k = 0.000, A₄ = −1.701E−01, A₆ = 3.174E−02, A₈ = 2.149E−03, A₁₀ = −7.002E−03, A₁₂ = 1.417E−03, A₁₄ = 2.060E−04, A₁₆ = −6.997E−05 Tenth Surface k = 0.000, A₄ = −1.692E−01, A₆ = 4.425E−02, A₈ = −1.136E−02, A₁₀ = 1.114E−03, A₁₂ = 2.619E−04, A₁₄ = −6.733E−05, A₁₆ = 3.353E−06 Eleventh Surface k = 0.000, A₄ = −1.720E−02, A₆ = 1.657E−02, A₈ = −3.265E−03, A₁₀ = 5.669E−04, A₁₂ = −7.870E−05, A₁₄ = 2.750E−06, A₁₆ = 3.348E−07 Twelfth Surface k = 0.000, A₄ = −2.717E−02, A₆ = 2.082E−02, A₈ = −5.708E−03, A₁₀ = 5.634E−04, A₁₂ = 1.272E−05, A₁₄ = −5.969E−06, A₁₆ = 2.546E−07 f1 = 90.92 mm f2 = 2.84 mm f3 = −5.75 mm f4 = 100.94 mm f5 = 101.70 mm f6 = −9.69 mm f56 = −11.75 mm F1 = 4.78 mm F2 = −12.37 mm The values of the respective conditional expressions are as follows:

-   f1/f=16.27 -   F1/f=0.86 -   f2/f=0.51 -   f3/f2=−2.02 -   F2/F1=−2.59 -   D34/f=0.12 -   f5/f4=1.01 -   f56/f=−2.10 -   f6/f56=0.83 -   f6/f=−1.73

Accordingly, the imaging lens of Numerical Data Example 7 satisfies the above-described conditional expressions. The distance on the optical axis X from the object-side surface of the first lens L1 to the image plane IM (length without the filter 10) is 6.06 mm, and downsizing of the imaging lens is attained.

FIG. 20 shows a lateral aberration that corresponds to the image height ratio H of the imaging lens, and FIG. 21 shows a spherical aberration (mm), astigmatism (mm), and a distortion (%), respectively, of the imaging lens of Numerical Data Example 7. As shown in FIGS. 20 and 21, according to the imaging lens of Numerical Data Example 7, the aberrations are also satisfactorily corrected.

According to the imaging lens of the embodiment described above, it is achievable to have a wide angle of view (2ω) of 80° or greater. According to Numerical Data Examples 1 to 7, the imaging lenses have wide angles of view of 64.2° to 80.0°. According to the imaging lens of the embodiment, it is possible to take an image over a wider range than that can be taken by a conventional imaging lens.

Moreover, in these years, with advancement in digital zoom technology, which enables to enlarge any area of an image obtained through an imaging lens by image processing, an imaging element having a high pixel count is often used in combination with a high resolution imaging lens. In case of such an imaging element with a high pixel count, a light-receiving area of each pixel decreases, so that an image taken tends to be dark. As a method for correcting this problem, there is a method of enhancing light-receiving sensitivity of the imaging element using an electrical circuit. However, when the light-receiving sensitivity increases, a noise component that does not directly contribute to image formation is also amplified, so that it is necessary to use another circuit for reducing the noise. According to the imaging lenses of Numerical Data Examples 1 to 7, the Fnos are as small as 2.2 to 3.0. According to the imaging lens of the embodiment, it is possible to obtain a sufficiently bright image without the above-described electrical circuit.

Accordingly, when the imaging lens of the embodiment is mounted in an imaging optical system such as cameras to be built in portable devices including cellular phones, portable information terminals, and smartphones, digital still cameras, security cameras, onboard cameras, and network cameras, it is possible to attain both high performance and downsizing of the cameras.

The present invention is applicable in an imaging lens to be mounted in a relatively small camera for portable devices including cellular phones, smartphones, and portable information terminals, digital still cameras, security cameras, onboard cameras, and network cameras.

The disclosure of Japanese Patent Application No. 2014-136541, filed on Jul. 2, 2014, is incorporated in the application by reference.

While the present invention has been explained with reference to the specific embodiment of the present invention, the explanation is illustrative and the present invention is limited only by the appended claims. 

What is claimed is:
 1. An imaging lens comprising: a first lens group; and a second lens group, arranged in this order from an object side to an image plane side, wherein said imaging lens has a total of six lenses, said first lens group includes a first lens having positive refractive power, a second lens having positive refractive power, and a third lens having at least one aspheric surface, these lenses arranged in this order from the object side to the image plane side and with a space in between, said second lens group includes a fourth lens having positive refractive power and at least one aspheric surface, a fifth lens having two aspheric surfaces, and a sixth lens having two aspheric surfaces, these lenses arranged in this order from the object side to the image plane side and with a space in between, said fifth lens is formed in a shape so that a curvature radius of the surface thereof on the image plane side is positive near an optical axis thereof, said sixth lens is formed in a shape so that a curvature radius of the surface thereof on the object side is negative near an optical axis thereof, and said second lens has a focal length f2 so that the following conditional expression is satisfied: 0.3<f2/f<0.9, where f is a focal length of a whole lens system.
 2. The imaging lens according to claim 1, wherein said second lens is formed in a shape so that a curvature radius of a surface thereof on the image plane side is negative near an optical axis thereof.
 3. The imaging lens according to claim 1, wherein said third lens is formed in a shape so that a curvature radius of a surface thereof on the object side and a curvature radius of a surface thereof on the image plane side are positive near an optical axis thereof.
 4. The imaging lens according to claim 1, wherein said sixth lens is formed in the shape so that a curvature radius of the surface thereof on the image plane side is negative near the optical axis thereof.
 5. The imaging lens according to claim 1, wherein said first lens has a focal length f1 so that the following conditional expression is satisfied: 10<f1/f<40.
 6. An imaging lens comprising: a first lens group; and a second lens group, arranged in this order from an object side to an image plane side, wherein said imaging lens has a total of six lenses, said first lens group includes a first lens having positive refractive power, a second lens having positive refractive power, and a third lens having at least one aspheric surface, these lenses arranged in this order from the object side to the image plane side and with a space in between, said second lens group includes a fourth lens having at least one aspheric surface, a fifth lens having negative refractive power and two aspheric surfaces, and a sixth lens having two aspheric surfaces, these lenses arranged in this order from the object side to the image plane side and with a space in between, said fifth lens is formed in a shape so that a curvature radius of the surface thereof on the image plane side is positive near an optical axis thereof, said sixth lens is formed in a shape so that a curvature radius of the surface thereof on the object side is negative near an optical axis thereof, and said second lens has a focal length f2 so that the following conditional expression is satisfied: 0.3<f2/f<0.9, where f is a focal length of a whole lens system.
 7. The imaging lens according to claim 6, wherein said second lens is formed in a shape so that a curvature radius of a surface thereof on the image plane side is negative near an optical axis thereof.
 8. The imaging lens according to claim 6, wherein said third lens is formed in a shape so that a curvature radius of a surface thereof on the object side and a curvature radius of a surface thereof on the image plane side are positive near an optical axis thereof.
 9. The imaging lens according to claim 6, wherein said sixth lens is formed in the shape so that a curvature radius of the surface thereof on the image plane side is negative near the optical axis thereof.
 10. The imaging lens according to claim 6, wherein said first lens has a focal length f1 so that the following conditional expression is satisfied: 10<f1/f<40.
 11. An imaging lens comprising: a first lens group; and a second lens group, arranged in this order from an object side to an image plane side, wherein said imaging lens has a total of six lenses, said first lens group includes a first lens having positive refractive power, a second lens having positive refractive power, and a third lens having at least one aspheric surface, these lenses arranged in this order from the object side to the image plane side and with a space in between, said second lens group includes a fourth lens having positive refractive power and at least one aspheric surface, a fifth lens having two aspheric surfaces, and a sixth lens having two aspheric surfaces, these lenses arranged in this order from the object side to the image plane side and with a space in between, said fifth lens is formed in a shape so that a curvature radius of the surface thereof on the image plane side is positive near an optical axis thereof, said sixth lens is formed in a shape so that a curvature radius of the surface thereof on the object side is negative near an optical axis thereof, and said third lens is arranged away from the fourth lens by a distance D34 on an optical axis thereof so that the following conditional expression is satisfied: 0.1<D34/f<0.4, where f is a focal length of a whole lens system.
 12. The imaging lens according to claim 11, wherein said second lens is formed in a shape so that a curvature radius of a surface thereof on the image plane side is negative near an optical axis thereof.
 13. The imaging lens according to claim 11, wherein said third lens is formed in a shape so that a curvature radius of a surface thereof on the object side and a curvature radius of a surface thereof on the image plane side are positive near an optical axis thereof.
 14. The imaging lens according to claim 11, wherein said sixth lens is formed in the shape so that a curvature radius of the surface thereof on the image plane side is negative near the optical axis thereof.
 15. The imaging lens according to claim 11, wherein said second lens has a focal length f2 and said third lens has a focal length f3 so that the following conditional expression is satisfied: −5<f3/f2<−1.
 16. An imaging lens comprising: a first lens group; and a second lens group, arranged in this order from an object side to an image plane side, wherein said imaging lens has a total of six lenses, said first lens group includes a first lens having positive refractive power, a second lens having positive refractive power, and a third lens having at least one aspheric surface, these lenses arranged in this order from the object side to the image plane side and with a space in between, said second lens group includes a fourth lens having at least one aspheric surface, a fifth lens having negative refractive power and two aspheric surfaces, and a sixth lens having two aspheric surfaces, these lenses arranged in this order from the object side to the image plane side and with a space in between, said fifth lens is formed in a shape so that a curvature radius of the surface thereof on the image plane side is positive near an optical axis thereof, said sixth lens is formed in a shape so that a curvature radius of the surface thereof on the object side is negative near an optical axis thereof, and said first lens group has a focal length F1 and said second lens group has a focal length F2 so that the following conditional expression is satisfied: −12<F2/F1<−1.5.
 17. The imaging lens according to claim 16, wherein said second lens is formed in a shape so that a curvature radius of a surface thereof on the image plane side is negative near an optical axis thereof.
 18. The imaging lens according to claim 16, wherein said third lens is formed in a shape so that a curvature radius of a surface thereof on the object side and a curvature radius of a surface thereof on the image plane side are positive near an optical axis thereof.
 19. The imaging lens according to claim 16, wherein said sixth lens is formed in the shape so that a curvature radius of the surface thereof on the image plane side is negative near the optical axis thereof.
 20. The imaging lens according to claim 16, wherein said fourth lens has a focal length f4 and said fifth lens has a focal length f5 so that the following conditional expression is satisfied: −15<f5/f4<−5. 